The numbers used to describe wrought aluminum alloys follow a code developed by the Aluminum Association. The first four digits describe the alloy; further digits or letters indicate the form of the product and the heat treatment it received.
Four-digit numerical designations
- 1st digit Alloy group
- 2nd digit Modifications in impurity limits
- 3rd & 4th Identify the alloys within a group
Alloy groups
- 1xxx Aluminum — 99.00% minimum
- 2xxx Copper
- 3xxx Manganese
- 4xxx Silicon
- 5xxx Magnesium
- 6xxx Magnesium & silicon
- 7xxx Zinc
- 8xxx Other elements
- 9xxx Unused series
The letter “X” before a four-digit number indicates an experimental alloy — for example X7004.
Tempers
Temper designations follow the alloy number.
- F As fabricated — no special control over the final temper
- O Annealed
- H Strain-hardened — strength increased by cold work
- T Thermally treated to produce stable tempers other than F, O or H
- W Solution heat-treated — an unstable temper
Subdivisions of the H temper
The first digit after the H gives the combination of basic operations:
- H1 Strain-hardened only
- H2 Strain-hardened and partially annealed
- H3 Strain-hardened and stabilized
- H4 Strain-hardened and lacquered or painted
The second digit gives the degree of strain-hardening:
- Hx1 Eighth hard
- Hx2 Quarter hard
- Hx4 Half hard
- Hx6 Three-quarter hard
- Hx8 Hard
- Hx9 Extra hard
A third digit is sometimes used when the control of temper or the mechanical properties differ from, but are close to, those of the two-digit H temper:
- H111 Strain-hardened less than the amount required for a controlled H11 temper
- H112 Some temper from shaping, without special control of strain-hardening or thermal treatment, but with mechanical property limits
- H311 Strain-hardened less than required for a controlled H31 temper
- H321 Strain-hardened less than required for a controlled H32 temper
- H323–H343 Specially fabricated for acceptable resistance to stress-corrosion cracking
Subdivisions of the T temper
- T1 Cooled from an elevated-temperature shaping process and naturally aged
- T2 Annealed (cast products only)
- T3 Solution heat-treated, then cold worked
- T4 Solution heat-treated and naturally aged
- T5 Cooled from an elevated-temperature shaping process, then artificially aged
- T6 Solution heat-treated, then artificially aged
- T7 Solution heat-treated, then over-aged / stabilized
- T8 Solution heat-treated, cold worked, then artificially aged
- T9 Solution heat-treated, artificially aged, then cold worked
- T10 Cooled from an elevated-temperature shaping process, cold worked, then artificially aged
Additional digits indicate a variation in treatment that significantly alters the characteristics of the product:
- Tx51 Stress-relieved by stretching
- Tx510 No further straightening after stretching
- Tx511 Minor straightening after stretching to meet standard tolerances
- Tx52 Stress-relieved by compressing